1. Field of the Invention
This invention relates to a method and an apparatus for modulation based on orthogonal frequency division multiplexing (OFDM). In addition, this invention relates to a method and an apparatus for demodulation concerning OFDM. Furthermore, this invention relates a method and an apparatus for digital quadrature modulation. Also, this invention relates to a method and an apparatus for digital quadrature demodulation.
2. Description of the Related Art
In orthogonal frequency division multiplexing (OFDM), digital in-phase (I) signals and digital quadrature (Q) signals resulting from a QAM-corresponding process or a QPSK-corresponding process are assigned to respective orthogonal frequencies for IDFT or IFFT. Here, QAM is short for quadrature amplitude modulation, and QPSK is short for quadrature phase shift keying. In addition, IDFT is short for inverse discrete Fourier transform, and IFFT is short for inverse fast Fourier transform. The IDFT or IFFT is executed while the I signals are set as real-part terms and the Q signals are set as imaginary-part terms. By the IDFT or IFFT, the I signals are converted and combined into a digital multiplexing-resultant I signal, and the Q signals are converted and combined into a digital multiplexing-resultant Q signal. The digital multiplexing-resultant I and Q signals are changed into analog forms. The analog I and Q signals are combined and converted into an RF multiple-carrier signal in a desired frequency band. The RF multiple-carrier signal is transmitted as a radio wave.
A prior-art apparatus for modulation based on OFDM requires a relatively-high sampling frequency. Therefore, the prior-art modulation apparatus uses parts which can accurately operate even at high frequencies. Such parts are expensive.
A prior-art apparatus for demodulation concerning OFDM requires a relatively-high sampling frequency. Therefore, the prior-art demodulation apparatus uses parts which can accurately operate even at high frequencies. Such parts are expensive.
It is a first object of this invention to provide an improved method of modulation based on orthogonal frequency division multiplexing (OFDM).
It is a second object of this invention to provide an improved apparatus for modulation based on OFDM.
It is a third object of this invention to provide an improved method of demodulation concerning OFDM.
It is a fourth object of this invention to provide an improved apparatus for demodulation concerning OFDM.
It is a fifth object of this invention to provide an improved digital quadrature modulation apparatus.
It is a sixth object of this invention to provide an improved method of digital quadrature modulation.
It is a seventh object of this invention to provide an improved digital quadrature demodulation apparatus.
It is an eighth object of this invention to provide an improved method of digital quadrature demodulation.
It is a ninth object of this invention to provide an improved digital quadrature modulator.
A first aspect of this invention provides a method of modulation based on orthogonal frequency division multiplexing. The method comprises the steps of assigning data pieces representative of in-phase components and quadrature components of a digital-modulation-resultant signal to frequencies for inverse fast Fourier transform; executing the inverse fast Fourier transform at a predetermined sampling frequency Fs to convert the data pieces into a real-part signal and an imaginary-part signal; shifting phases of the real-part signal and the imaginary-part signal to convert the real-part signal and the imaginary-part signal into a phase-shifted real-part signal and a phase-shifted imaginary-part signal; dividing the phase-shifted real-part signal into a sequence of even-numbered samples and a sequence of odd-numbered samples; dividing the phase-shifted imaginary-part signal into a sequence of even-numbered samples and a sequence of odd-numbered samples; multiplying the sequence of the phase-shifted even-numbered samples of the real-part signal by xe2x80x9c1xe2x80x9d to generate a first multiplication-result signal I(2n); multiplying the sequence of the even-numbered samples of the phase-shifted imaginary-part signal by xe2x80x9cxe2x88x921xe2x80x9d to generate a second multiplication-result signal xe2x88x92Q(2n); multiplying the sequence of the odd-numbered samples of the phase-shifted real-part signal by xe2x80x9cxe2x88x921xe2x80x9d to generate a third multiplication-result signal xe2x88x92I(2n+1); multiplying the sequence of the odd-numbered samples of the phase-shifted imaginary-part signal by xe2x80x9c1xe2x80x9d to generate a fourth multiplication-result signal Q(2n+1); sequentially selecting the first multiplication-result signal I(2n), the second multiplication-result signal xe2x88x92Q(2n), the third multiplication-result signal xe2x88x92I(2n+1), and the fourth multiplication-result signal Q(2n+1) at a frequency equal to twice the predetermined sampling frequency Fs to generate a digital quadrature-modulation-resultant signal; and converting the digital quadrature-modulation-resultant signal into an analog quadrature-modulation-resultant signal at a frequency equal to twice the predetermined sampling frequency Fs.
A second aspect of this invention is based on the first aspect thereof, and provides a method wherein the sequentially selecting step comprises inputting the first multiplication-result signal I(2n), the second multiplication-result signal xe2x88x92Q(2n), the third multiplication-result signal xe2x88x92I(2n+1), and the fourth multiplication-result signal Q(2n+1) into shift registers respectively at a frequency equal to half the predetermined sampling frequency Fs; and sequentially selecting output signals from the shift registers at a frequency equal to twice the predetermined sampling frequency Fs to generate the digital quadrature-modulation-resultant signal.
A third aspect of this invention provides an apparatus for modulation based on orthogonal frequency division multiplexing. The apparatus comprises means for assigning data pieces representative of in-phase components and quadrature components of a digital-modulation-resultant signal to frequencies for inverse fast Fourier transform; means for executing the inverse fast Fourier transform at a predetermined sampling frequency Fs to convert the data pieces into a real-part signal and an imaginary-part signal; means for shifting phases of the real-part signal and the imaginary-part signal to convert the real-part signal and the imaginary-part signal into a phase-shifted real-part signal and a phase-shifted imaginary-part signal; means for dividing the phase-shifted real-part signal into a sequence of even-numbered samples and a sequence of odd-numbered samples; means for dividing the phase-shifted imaginary-part signal into a sequence of even-numbered samples and a sequence of odd-numbered samples; a multiplier for multiplying the sequence of the even-numbered samples of the phase-shifted real-part signal by xe2x80x9c1xe2x80x9d to generate a first multiplication-result signal I(2n); a multiplier for multiplying the sequence of the even-numbered samples of the phase-shifted imaginary-part signal by xe2x80x9cxe2x88x921xe2x80x9d to generate a second multiplication-result signal xe2x88x92Q(2n); a multiplier for multiplying the sequence of the odd-numbered samples of the phase-shifted real-part signal by xe2x80x9cxe2x88x921xe2x80x9d to generate a third multiplication-result signal xe2x88x92I(2n+1); a multiplier for multiplying the sequence of the odd-numbered samples of the phase-shifted imaginary-part signal by xe2x80x9c1xe2x80x9d to generate a fourth multiplication-result signal Q(2n+1); means for sequentially selecting the first multiplication-result signal I(2n), the second multiplication-result signal xe2x88x92Q(2n), the third multiplication-result signal xe2x88x92I(2n+1), and the fourth multiplication-result signal Q(2n+1) at a frequency equal to twice the predetermined sampling frequency Fs to generate a digital quadrature-modulation-resultant signal; and a D/A converter for converting the digital quadrature-modulation-resultant signal into an analog quadrature-modulation-resultant signal at a frequency equal to twice the predetermined sampling frequency Fs.
A fourth aspect of this invention is based on the third aspect thereof, and provides an apparatus wherein the sequentially selecting means comprises shift registers; means for inputting the first multiplication-result signal I(2n), the second multiplication-result signal xe2x88x92Q(2n), the third multiplication-result signal xe2x88x92I(2n+1), and the fourth multiplication-result signal Q(2n+1) into the shift registers respectively at a frequency equal to half the predetermined sampling frequency Fs; and a data selector for sequentially selecting output signals from the shift registers at a frequency equal to twice the predetermined sampling frequency Fs to generate the digital quadrature-modulation-resultant signal.
A fifth aspect of this invention provides a method of demodulating an OFDM (orthogonal frequency division multiplexed) signal. The method comprises the steps of converting an RF OFDM signal into an analog OFDM signal in a frequency band centered at a half of a predetermined sampling frequency Fs; converting the analog OFDM signal into a digital OFDM signal at a frequency equal to twice the predetermined sampling frequency Fs; sequentially separating samples of the digital OFDM signal into a sequence of first every four samples, a sequence of second every four samples, a sequence of third every four samples, and a sequence of fourth every four samples; multiplying the sequence of the first every four samples of the digital OFDM signal by xe2x80x9c1xe2x80x9d to generate a first multiplication-result signal I(2n); multiplying the sequence of the second every four samples of the digital OFDM signal by xe2x80x9cxe2x88x921xe2x80x9d to generate a second multiplication-result signal xe2x88x92Q(2n); multiplying the sequence of the third every four samples of the digital OFDM signal by xe2x80x9cxe2x88x921xe2x80x9d to generate a third multiplication-result signal xe2x88x92I(2n+1); multiplying the sequence of the fourth every four samples of the digital OFDM signal by xe2x80x9c1xe2x80x9d to generate a fourth multiplication-result signal Q(2n+1); generating a quadrature-demodulation-resultant in-phase signal in response to the first multiplication-result signal I(2n) and the third multiplication-result signal xe2x88x92I(2n+1); generating a quadrature-demodulation-resultant quadrature signal in response to the second multiplication-result signal xe2x88x92Q(2n) and the fourth multiplication-result signal Q(2n+1); equalizing phases of the quadrature-demodulation-resultant in-phase signal and the quadrature-demodulation-resultant quadrature signal to generate a pair of equal-phase in-phase and quadrature signals; and subjecting the pair of the equal-phase in-phase and quadrature signals to fast Fourier transform at a frequency equal to the predetermined sampling frequency to convert the pair of the equal-phase in-phase and quadrature signals into real-part data and imaginary-part data.
A sixth aspect of this invention provides an apparatus for demodulating an OFDM (orthogonal frequency division multiplexed) signal. The apparatus comprises a frequency converter for converting an RF OFDM signal into an analog OFDM signal in a frequency band centered at a half of a predetermined sampling frequency Fs; an A/D converter for converting the analog OFDM signal into a digital OFDM signal at a frequency equal to twice the predetermined sampling frequency Fs; means for sequentially separating samples of the digital OFDM signal into a sequence of first every four samples, a sequence of second every four samples, a sequence of third every four samples, and a sequence of fourth every four samples; a multiplier for multiplying the sequence of the first every four samples of the digital OFDM signal by xe2x80x9c1xe2x80x9d to generate a first multiplication-result signal I(2n); a multiplier for multiplying the sequence of the second every four samples of the digital OFDM signal by xe2x80x9cxe2x88x921xe2x80x9d to generate a second multiplication-result signal xe2x88x92Q(2n); a multiplier for multiplying the sequence of the third every four samples of the digital OFDM signal by xe2x80x9cxe2x88x921xe2x80x9d to generate a third multiplication-result signal xe2x88x92I(2n+1); a multiplier for multiplying the sequence of the fourth every four samples of the digital OFDM signal by xe2x80x9c1xe2x80x9d to generate a fourth multiplication-result signal Q(2n+1); means for generating a quadrature-demodulation-resultant in-phase signal in response to the first multiplication-result signal I(2n) and the third multiplication-result signal xe2x88x92I(2n+1); means for generating a quadrature-demodulation-resultant quadrature signal in response to the second multiplication-result signal xe2x88x92Q(2n) and the fourth multiplication-result signal Q(2n+1); means for equalizing phases of the quadrature-demodulation-resultant in-phase signal and the quadrature-demodulation-resultant quadrature signal to generate a pair of equal-phase in-phase and quadrature signals; and means for subjecting the pair of the equal-phase in-phase and quadrature signals to fast Fourier transform at a frequency equal to the predetermined sampling frequency to convert the pair of the equal-phase in-phase and quadrature signals into real-part data and imaginary-part data.
A seventh aspect of this invention provides a digital quadrature modulation apparatus comprising means for generating N-point multiple-carrier signals having a predetermined sampling frequency Fs and being in a frequency band whose center frequency is equal to 0 Hz, the N-point multiple-carrier signals being in sets each having an in-phase component and a quadrature component, where N denotes a predetermined natural number; means for subjecting the N-point multiple-carrier signals to inverse fast Fourier transform to generate a discrete in-phase signal I and a discrete quadrature signal Q defined in a time domain; means for selecting samples I(2n) and I(2n+1) of the discrete in-phase signal I and samples Q(2n) and Q(2n+1) of the discrete quadrature signal Q, where n=0, 1, 2, . . . , N/2; means for multiplying the selected sample I(2n+1) by xe2x80x9cxe2x88x921xe2x80x9d to generate a sample xe2x88x92I(2n+1); means for multiplying the selected sample Q(2n) by xe2x80x9cxe2x88x921xe2x80x9d to generate a sample xe2x88x92Q(2n); and means for rearranging the samples I(2n), xe2x88x92I(2n+1), xe2x88x92Q(2n), and Q(2n+1) in an order as I(2n), xe2x88x92Q(2n), xe2x88x92I(2n+1), and Q(2n+1) to generate a quadrature-modulation-resultant signal having a sampling frequency equal to twice the predetermined sampling frequency Fs and being in a frequency band whose center frequency is equal to a half of the predetermined sampling frequency Fs.
An eighth aspect of this invention provides a method of digital quadrature modulation. The method comprises the steps of generating N-point multiple-carrier signals having a predetermined sampling frequency Fs and being in a frequency band whose center frequency is equal to 0 Hz, the N-point multiple-carrier signals being in sets each having an in-phase component and a quadrature component, where N denotes a predetermined natural number; subjecting the N-point multiple-carrier signals to inverse fast Fourier transform to generate a discrete in-phase signal I and a discrete quadrature signal Q defined in a time domain; selecting samples I(2n) and I(2n+1) of the discrete in-phase signal I and samples Q(2n) and Q(2n+1) of the discrete quadrature signal Q, where n=0, 1, 2, . . . , N/2; multiplying the selected sample I(2n+1) by xe2x80x9cxe2x88x921xe2x80x9d to generate a sample xe2x88x92I(2n+1); multiplying the selected sample Q(2n) by xe2x80x9cxe2x88x921xe2x80x9d to generate a sample xe2x88x92Q(2n); and rearranging the samples I(2n), xe2x88x92I(2n+1), xe2x88x92Q(2n), and Q(2n+1) in an order as I(2n), xe2x88x92Q(2n), xe2x88x92I(2n+1), and Q(2n+1) to generate a quadrature-modulation-resultant signal having a sampling frequency equal to twice the predetermined sampling frequency Fs and being in a frequency band whose center frequency is equal to a half of the predetermined sampling frequency Fs.
A ninth aspect of this invention provides a digital quadrature demodulation apparatus for demodulating a quadrature-modulation-resultant signal generated by the digital quadrature modulation apparatus according to the seventh aspect of this invention. The digital quadrature demodulation apparatus comprises means for selecting four successive samples of 2N-point discrete data being a quadrature-modulation-resultant signal which has a sampling frequency equal to twice the predetermined sampling frequency Fs and which is in a frequency band whose center frequency is equal to a half of the predetermined sampling frequency Fs; and means for sequentially assigning the selected four successive samples to a sample I(2n) of an in-phase signal, an inversion xe2x88x92Q(2n) of a sample Q(2n) of a quadrature signal, an inversion xe2x88x92I(2n+1) of a sample I(2n+1) of the in-phase signal, and a sample Q(2n+1) of the quadrature signal, respectively, wherein each of the in-phase signal and the quadrature signal has N points and is in a frequency band whose center frequency is equal to 0 Hz.
A tenth aspect of this invention provides a method of demodulating a quadrature-modulation-resultant signal generated by the method according to the eighth aspect of this invention. The demodulating method comprises the steps of selecting four successive samples of 2N-point discrete data being a quadrature-modulation-resultant signal which has a sampling frequency equal to twice the predetermined sampling frequency Fs and which is in a frequency band whose center frequency is equal to a half of the predetermined sampling frequency Fs; and sequentially assigning the selected four successive samples to a sample I(2n) of an in-phase signal, an inversion xe2x88x92Q(2n) of a sample Q(2n) of a quadrature signal, an inversion xe2x88x92I(2n+1) of a sample I(2n+1) of the in-phase signal, and a sample Q(2n+1) of the quadrature signal, respectively, wherein each of the in-phase signal and the quadrature signal has N points and is in a frequency band whose center frequency is equal to 0 Hz.
An eleventh aspect of this invention provides a digital quadrature modulator comprising an interpolation filter for generating second samples of an N-point quadrature signal from original samples of the quadrature signal by an interpolation process, where N denotes a predetermined natural number, the second samples being at centers between positions of the original samples; a first demultiplexer for separating samples of an N-point in-phase signal into a sequence of even-numbered samples and a sequence of odd-numbered samples, wherein the sequence of the even-numbered samples of the in-phase signal compose a signal I(2n); a second demultiplexer for separating the second samples of the quadrature signal into a sequence of even-numbered samples and a sequence of odd-numbered samples, wherein the sequence of the odd-numbered samples of the quadrature signal compose a signal Q(2n+1); a first multiplier for multiplying the sequence of the odd-numbered samples of the in-phase signal by xe2x80x9cxe2x88x921xe2x80x9d to generate a signal xe2x88x92I(2n+1); a second multiplier for multiplying the sequence of the even-numbered samples of the quadrature signal by xe2x80x9cxe2x88x921xe2x80x9d to generate a signal xe2x88x92Q(2n); and a parallel-to-serial converter for sequentially selecting and outputting the signal I(2n), the signal xe2x88x92Q(2n), the signal xe2x88x92I(2n+1), and the signal Q(2n+1) to generate a 2N-point digital quadrature-modulation-resultant signal.
A twelfth aspect of this invention provides a digital quadrature modulator comprising an interpolation filter for generating second samples of an N-point in-phase signal from original samples of the in-phase signal by an interpolation process, where N denotes a predetermined natural number, the second samples being at centers between positions of the original samples; a first demultiplexer for separating the second samples of the in-phase signal into a sequence of even-numbered samples and a sequence of odd-numbered samples, wherein the sequence of the even-numbered samples of the in-phase signal compose a signal I(2n); a second demultiplexer for separating samples of an N-point quadrature signal into a sequence of even-numbered samples and a sequence of odd-numbered samples, wherein the sequence of the odd-numbered samples of the quadrature signal compose a signal Q(2n+1); a first multiplier for multiplying the sequence of the odd-numbered samples of the in-phase signal by xe2x80x9cxe2x88x921xe2x80x9d to generate a signal xe2x88x92I(2n+1); a second multiplier for multiplying the sequence of the even-numbered samples of the quadrature signal by xe2x80x9cxe2x88x921xe2x80x9d to generate a signal xe2x88x92Q(2n); and a parallel-to-serial converter for sequentially selecting and outputting the signal I(2n), the signal xe2x88x92Q(2n), the signal xe2x88x92I(2n+1), and the signal Q(2n+1) to generate a 2N-point digital quadrature-modulation-resultant signal.
A thirteenth aspect of this invention is based on the eleventh aspect thereof, and provides a digital quadrature modulator wherein the interpolation filter comprises a half band filter.
A fourteenth aspect of this invention provides a method of digital quadrature modulation. The method comprises the steps of generating second samples of an N-point quadrature signal from original samples of the quadrature signal by an interpolation process, where N denotes a predetermined natural number, the second samples being at centers between positions of the original samples; separating samples of an N-point in-phase signal into a sequence of even-numbered samples and a sequence of odd-numbered samples, wherein the sequence of the even-numbered samples of the in-phase signal compose a signal I(2n); separating the second samples of the quadrature signal into a sequence of even-numbered samples and a sequence of odd-numbered samples, wherein the sequence of the odd-numbered samples of the quadrature signal compose a signal Q(2n+1); multiplying the sequence of the odd-numbered samples of the in-phase signal by xe2x80x9cxe2x88x921xe2x80x9d to generate a signal xe2x88x92I(2n+1); multiplying the sequence of the even-numbered samples of the quadrature signal by xe2x80x9cxe2x88x921xe2x80x9d to generate a signal xe2x88x92Q(2n); and sequentially selecting and outputting the signal I(2n), the signal xe2x88x92Q(2n), the signal xe2x88x92I(2n+1), and the signal Q(2n+1) to generate a 2N-point digital quadrature-modulation-resultant signal.
A fifteenth aspect of this invention provides a method of digital quadrature modulation. The method comprises the steps of generating second samples of an N-point in-phase signal from original samples of the in-phase signal by an interpolation process, where N denotes a predetermined natural number, the second samples being at centers between positions of the original samples; separating the second samples of the in-phase signal into a sequence of even-numbered samples and a sequence of odd-numbered samples, wherein the sequence of the even-numbered samples of the in-phase signal compose a signal I(2n); separating samples of an N-point quadrature signal into a sequence of even-numbered samples and a sequence of odd-numbered samples, wherein the sequence of the odd-numbered samples of the quadrature signal compose a signal Q(2n+1); multiplying the sequence of the odd-numbered samples of the in-phase signal by xe2x80x9cxe2x88x921xe2x80x9d to generate a signal xe2x88x92I(2n+1); multiplying the sequence of the even-numbered samples of the quadrature signal by xe2x80x9cxe2x88x921xe2x80x9d to generate a signal xe2x88x92Q(2n); and sequentially selecting and outputting the signal I(2n), the signal xe2x88x92Q(2n), the signal xe2x88x92I(2n+1), and the signal Q(2n+1) to generate a 2N-point digital quadrature-modulation-resultant signal.